RELEASE: 92-46
NASA'S HUBBLE FINDS NEW EVIDENCE FOR MASSIVE BLACK HOLES
Astronomers report that they have found new evidence
that a black hole, weighing 3 million times the mass of the
Sun, exists at the center of the nearby elliptical galaxy
M32, based on images obtained with NASA's Hubble Space
Telescope (HST). The images show that the stars in M32
become extremely concentrated toward the nucleus.
This central structure resembles the gravitational
"signature" of a massive black hole. The presence of a
black hole in an ordinary galaxy like M32 may mean that
inactive black holes are common to the centers of galaxies.
The new HST images show that M32 is an interesting
"laboratory" for testing theories of the formation of
massive black holes.
This result is based on image analysis conducted by Dr.
Tod R. Lauer of the National Optical Astronomy
Observatories, Tuscon, Ariz., Dr. Sandra M. Faber of the
University of California, Santa Cruz, and other members of
the HST Wide Field/Planetary Camera (WFPC) Imaging Team.
M32 is quite small and compact as elliptical galaxies
go, containing about 400 million stars within a diameter of
only 1,000 light-years. At a distance of 2.3 million
light-years, M32 (the 32nd object in a catalog of
non-stellar objects compiled by French astronomer Charles
Messier in 1774) is one of the closest neighbors to the
Milky Way galaxy.
M32 is a satellite of the great spiral galaxy in
Andromeda M31, which dominates the small group of galaxies
of which the Milky Way is a member. M31 can be seen with
the naked eye as a spindle-shaped "cloud" the width of the
full moon, and its small companion M32 can be seen with a
small telescope.
M32 has been among the best candidates for a galaxy with
a massive central black hole. This was first proposed in
1987 by Dr. John L. Tonry of the Massachusetts Institute of
Technology, Cambridge, and independently by Dr. Alan
Dressler of the Observatories of the Carnegie Institution,
Wash., and by Dr. Douglas O. Richstone of the University of
Michigan, Ann Arbor.
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Their observations, made with ground-based telescopes,
showed an abrupt increase in the orbital velocities of stars
towards the center of M32. This data led the astronomers
to conclude that M32 must have a strong but unseen
concentration of mass at its center. A black hole at least
several million times the mass of the Sun is the most likely
type of object matching these characteristics.
Ground-based images, however, do not have enough
resolution to detect the effects of a massive black hole on
the structure of M32. The HST images analyzed by Lauer,
Faber and co-investigators on the WFPC imaging team now show
the nucleus of M32 in clear detail.
They find that the density of stars in the nucleus of
M32 appears to increase steadily towards the center, with no
sign of leveling off. These results are very similar to the
predictions for what a massive black hole should do to the
central structure of a galaxy.
"This is the densest stellar system known to
astronomers," says Lauer. "The density of stars at the
center of M32 may be over 100 million times greater than the
distribution of stars in the neighborhood of the Sun. A
visitor to a planet at the center of M32 would see a starry
night sky so saturated with stars that their combined light
would be brighter than 100 full moons. The night would
never get darker than mid-twilight on the Earth, and one
could even read a newspaper by starlight."
To date, HST has uncovered the gravitational signature
of a black hole in one other galaxy, the giant elliptical
galaxy called M87. Both M32 and M87 have a distinctive
central concentration of starlight called a "cusp." These
two galaxies are quite different, though, in that the black
hole proposed for M32 is roughly 1,000 times smaller than
the black hole that might exist at the heart of M87.
Although M32 is about 20 times closer to the Earth than
M87, its much smaller black hole means that the brightness
cusp also is much smaller and is close to HST's resolution
limits. Unlike M87, M32 lacks any form of nuclear activity,
which means that at present the black hole would not be
accreting significant amounts of matter. This result also
raises the possibility that small inactive black holes are
common to the centers of galaxies.
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Because the region dominated by the black hole is so
small, Lauer and Faber also considered the possibility that
no black hole is present. Instead, the star density might
level off just beyond the resolution limits of HST. If this
were the case for M32, it would force the researchers to
conclude that the center of M32 is unstable and vulnerable
to collapse.
A black hole at the center of M32 would have the
paradoxical effect of stabilizing the galaxy's nucleus.
That's because the stars orbit so rapidly around the black
hole, they move past each other too quickly to
gravitationally capture each other or collide. The black
hole thus keeps the center of a galaxy "stirred up."
In the absence of a black hole, however, the stars would
move slowly enough to attract each other gravitationally.
Collisions between stars become much more frequent, and
heavier slower moving stars sink to the center of the galaxy
causing it to collapse.
The fate of the collapsing core is uncertain. One
possibility is that binary stars formed during the collapse
would provide enough kinetic energy to halt the collapse by
transferring momentum to single stars. This would make the
core rebound, like a rubber ball that has been squeezed and
then relaxed.
An alternative possibility is that runaway merging of
stars would occur during core collapse, leading to the
formation of a black hole in any case. If so, this would
rule out alternative explanations that don't require a black
hole.
If the core is really unstable, the researchers would
expect to find evidence of merged and captured stars called
"blue stragglers'' (HST has in fact uncovered such stars at
the core of a globular cluster, a much smaller aggregate of
stars than M32). The shape of the starlight distribution at
the core would also be different from that which HST
detects.
The Hubble images instead show that the population of
stars in the nucleus is the same as that further out in the
galaxy, and that the shape of M32 remains constant into the
center. This means that a core collapse has not recently
occurred.
At the present time, astrophysical theories are not
sophisticated enough to say whether or not M32 would have to
evolve to make a central black hole, but do raise this as an
intriguing possibility. The new HST observations thus
identify M32 as an interesting "laboratory" where
astronomers can test theories of massive black hole
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The search for super massive black holes in the cores of
galaxies is one of the primary missions of NASA's Hubble
Space Telescope. By investigating both active and quiescent
galaxies, astronomers will have a better idea of the
conditions and events which lead to the formation and growth
of super-massive black holes.
The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc.,
for NASA, under contract with the Goddard Space Flight
Center, Greenbelt, Md. The Hubble Space Telescope is a
project of international cooperation between NASA and the
European Space Agency.
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